The field of Developmental Genetics is broad. It encompasses biochemical and molecular as well as classical developmental approaches. The latter type of approach is pursued in our laboratory. Much of the work in developmental genetics consists of relatively independent investigations of specific phenomena. The main research proposed in this application continues to be directed to the analysis of pattern in the development of Drosophila by the use of mutant genotypes in genetically homogeneous individuals and in genetic mosaics obtained primarily as a result of somatic crossing over induced by X-rays. It also provides for exploration of developmental-genetic problems which are concerned with other aspects of gene action. In each case an attempt is made to relate the specific study to general aspects of differentiation and gene action. Since much of the work rests on the production of genetic mosaics it seems appropriate to study mechanisms of mosaic production particularly the mechanism of somatic recombination.